1. Field of the Invention
The present invention relates to a method for manufacturing a semiconductor device including a circuit made by using thin film transistors (hereinafter, TFT). For example, the present invention relates to a method for manufacturing a light-emitting device comprising a light-emitting element, which emits fluorescence or phosphorescence upon application of an electric field thereto. The light-emitting element includes a pair of electrodes and a layer containing organic compound (hereinafter, an organic compound layer) that is sandwiched between the pair of electrodes. In this specification, the term light-emitting device includes an image display device, a device for emitting light, or a light source (including a lighting system). Also, the following modules are included in the light-emitting device: a module obtained by attaching a connector such as an FPC (flexible printed circuit), a TAB (tape automated bonding) tape, or a TCP (tape carrier package) to a light-emitting element; a module obtained by providing a tip of a TAB tape or a TCP with a printed wiring board; and a module obtained by directly mounting an IC (integrated circuit) to a light-emitting element by the COG (chip on glass) system.
2. Description of the Related Art
(EL Element)
In recent years, study of a light-emitting device having an EL element as a self-luminous element has become vigorous. In particular, a light-emitting device using an organic material as an EL material has attracted an attention. The light-emitting device is also referred to as an EL display.
The light-emitting device has no limitation in angle of visibility because the light-emitting device is a self-luminous type, which is different from a liquid crystal display device. In other words, the light-emitting device is superior to a liquid crystal display as a display to be used in the open air, and usages thereof in various ways have been proposed.
The EL element includes an organic compound layer that provides an electro luminescence generated by being applied with an electric field (hereinafter referred to as EL layer), an anode, and a cathode. The luminescence of the organic compound includes light emission (fluorescence) generated when restoring from a singlet excitation state to the ground state, and light emission (phosphorescence) when restoring from a triplet excitation state to the ground state. The light-emitting device manufactured by a film-forming device and a method of forming a film according to the present invention may be applied to both of the cases employing these light-emissions.
A light-emitting element comprising a cathode, an EL layer, and an anode is referred to as an EL element in this specification. There are two methods of forming the EL elements, a method of forming an EL layer between two types of stripe shape electrodes that are formed in such a way that they can mutually intersect (simple matrix method), and a method of forming EL layers between pixel electrodes and opposing electrodes that are disposed in a matrix and are connected to TFTs (active matrix method). However, when the pixel density increases, it is considered preferable to use the active matrix type in which a switch is provided for every pixel (or every dot) since it can be driven at a low voltage.
(Sealing)
An EL material to form an EL layer deteriorates extremely easily, and is easily oxidized or absorbs moisture due to oxygen or moisture. Thus, a light-emitting element has a problem of a decline in luminance or short lifetime.
Conventionally, a light-emitting element is covered with an encapsulating can, the inside is filled with dried nitrogen gas, and a drying agent is further adhered thereto, thereby preventing oxygen or moisture from reaching the light-emitting element.
A thermosetting sealing material and a UV curing material are mainly used as a sealing material for sealing elements. A two-component sealing material is mixed with air bubbles in mixing, a time before bonding after mixing is limited, working efficiency is poor, and thus the two-component sealing material is not used so often.
Although the thermosetting material is cured by heating, a whole element is exposed to high heat for curing, and thus characteristics of the element is deteriorated. It is difficult to enhance productivity, since the time for curing is long. On the other hand, in the case of UV curing material, the speed of curing is high, and productivity can be easily enhanced, an element to be used is not damaged by heat, and thus, the UV curing material is widely used as a sealing material for sealing an EL element (Reference 1: Japanese Patent Application Laid Open No. 2001-139933).
The conventional light-emitting device has the structure that has a light-emitting element in which an electrode on a substrate is formed as an anode, an organic compound layer is formed on the anode, and a cathode is formed on the organic compound layer, and light generated in the organic compound layer is emitted through the anode formed as a transparent electrode toward a TFT (hereinafter, the structure is referred to as a bottom emission). In such a bottom emission structure, there is a problem that aperture ratio of a pixel is limited by a structure of a TFT circuit.
On the other hand, in the case of a structure in which an anode is formed on a substrate, an organic compound layer is formed on the anode, and a cathode as a transparent electrode is formed on the organic compound layer (hereinafter, the structure is referred to as a top emission), the aperture ratio of a pixel is larger than the bottom emission.
It is, however, difficult to arrange a drying agent over a pixel portion, or use an encapsulating can formed of materials that can shield a display light, since the drying agent or the encapsulating can disturbs the display in the top emission structure. Further, since the drying agent is highly hygroscopic, the drying agent requires careful handling and quick operation in sealing.
In the case of the top emission structure in which a substrate, with light-emitting elements formed thereupon, is bonded to a sealing substrate, a space over the pixel portion is filled and sealed with a first transparent sealing material in bonding the two substrates, and thus the whole pixel portion is covered with the sealing material, thereby suppressing diffusion of oxygen or water molecule into the element.
It is possible to employ a structure in which the first sealing material is surrounded by a second sealing material (having a higher viscosity than the first sealing material) that contains a gap material (filler, fine particles, or the like) for maintaining a space between the two substrates. At the time, the first sealing material and the second sealing material thus seal the light-emitting element.
There is, however, a fear that air bubbles spreading to the pixel portion will remain in corners when a seal pattern shape for the second sealing material is formed into a square shape, an inverted “c” shape, or a “U” shape, and the two substrates are bonded by dripping the first sealing material having low viscosity thereon.
Although the UV curing sealing method is suitable for sealing an element as described above, the seal pattern has a limitation, when there is a portion (such as a wiring portion of a TFT substrate or a sealing can) through which UV light cannot pass. Particularly, in the element in which a sealing substrate having a color filter is bonded to the top emission element, it is impossible to fill and seal the space over the pixel portion with UV curing sealing material, since neither a TFT substrate nor the sealing substrate having a color filter can transmit UV light. Even if the sealing substrate can transmit UV light, it is necessary to give extra consideration in order not to irradiate the EL layer with the UV light that is a high-energy line. In the top emission element in which the transparent electrode exists in a lower portion of the sealing material, the UV light reaches the EL layer, and thus the element is damaged.